1. Özkan ŞAHİN & Tadeusz KOWALSKI Uludağ University, Physics Department, Bursa – TURKEY Faculty of Physics and Applied Computer Science, AGH University of Science and Technology, Krakow – POLAND Recent gain calculations

2. Gain measurements and fits for Ar – CO 2 mixtures 2/14  High precision gain measurements in Krakow (Tadeusz KOWALSKI)  special guard for dark current  no need to use gain scaling  Wide gain regime: ionisation to gain of 10 5 ; less than 5% error on gas gain  Pressure range: 0.4 – 1.8 atm  Admixture concentration: 1 – 50 % CO 2 MPGD 2013 & 12th RD51 Collaboration Meeting 14– 17 October 2013, CERN

3. Extracted transfer rates 3/14  Fit function for 1%, 2%, 4% CO 2  Scaled fits: normalised to 50% CO 2, using its fit parameters (plot on the left)  Fit function for 5.73%, 30%, 50% CO 2 MPGD 2013 & 12th RD51 Collaboration Meeting 14– 17 October 2013, CERN  p 2 dependence: evidence of 3–body interactions

4. Modelling of the transfer rates 4/14 MPGD 2013 & 12th RD51 Collaboration Meeting 14– 17 October 2013, CERN  Combined fit (concentration + pressure)  All transfer rates included: Green curves extracted from T.Z. Kowalski et al. NIM A 323 (1992) 289–293  Confirmation of the transfer curves with earlier data at 1070 hPa  Lower uncertainty with recent data  Positive radiative term at low CO2 concentrations From recent gain measurements [Ö. Şahin et al. JINST P05002 (2010) 1–30] Plotted by Rob Veenhof

5. Pressure and concentration depence of feedback 5/14 MPGD 2013 & 12th RD51 Collaboration Meeting 14– 17 October 2013, CERN  1% CO 2 : increase with gas pressure  higher  for 24  m anode radius  2%, 4%, 5.73% CO 2 : decrease on  with pressure  0.4 atm: almost flat  for the mixtures with 2%, 4% and 5.73% CO 2  0.8 atm – 1.2 atm:  decrease with CO 2 concentrations Decreases of  easy to explain in terms of photon mean free path !!!

6. Photo – absorption cross section of CO 2 6/14 MPGD 2013 & 12th RD51 Collaboration Meeting 14– 17 October 2013, CERN  photons from 3d and higher radiative levels can ionise CO 2  4s photons produce photo – electron if they arrive the cathode but they can not ionise CO 2  non – radiative states decays to intermediate states; they have not enough energy to ionise CO 2 or to extract electron from cathode  σ pa : 3d, 5s > 4s > 4d Cross section compiled from J. Berkowitz, Atomic and Molecular Photoabsorption, Chapter 5, p. 189–197, Academic Press (2002)

7. Production rates and avalanche region 7/14 MPGD 2013 & 12th RD51 Collaboration Meeting 14– 17 October 2013, CERN  4s levels are the most abudantly produced  they are not lost in Penning transfers  they can contribute to feedback effectively by arriving the cathode ???  α = 1 used to define avalanche sizes

8. Absorption distance of the excited states 8/14 MPGD 2013 & 12th RD51 Collaboration Meeting 14– 17 October 2013, CERN  Both avalanche sizes and photon mean free path decreases with pressure and size reduction is smaller than the mean free path  photons in 1% and 2% CO 2 stopped outside the avalanche (still in gas) while the rest are absorbed in the avalanche  Size in the tube with thinner anode wire is smaller: explains bigger beta for r a = 24  m Gain: 80 – 8000 Gain: 2000 – 8000 r a = 50  m

9. Gain in Ne – CO 2 mixtures 9/14 MPGD 2013 & 12th RD51 Collaboration Meeting 14– 17 October 2013, CERN  Measured by Tadeusz KOWALSKI with the same technique and equipment  Present calculations cover 2% – 20 % CO 2 mixtures  All excited states of Ne can ionise CO 2 (3d and higher in Ar – CO 2 mixtures)  No gain scaling needed as before

10. Transfer rates (0.4 and 0.8 atm) 10/14 MPGD 2013 & 12th RD51 Collaboration Meeting 14– 17 October 2013, CERN  the biggest transfer at the lowest CO 2 concentration  bump in 10% CO2 mixture then sharp decrease for higher fractions  larger rate for the tube with thicker anode wire (r a = 50  m)  rates are almost flat including 10% CO 2  then they decrease in 15% and 20% CO 2  the rates are bigger at 0.8 atm  errors on transfer rates are getting bigger for high concentrations

11. Transfer rates (1.2 and 1.8 atm) 11/14 MPGD 2013 & 12th RD51 Collaboration Meeting 14– 17 October 2013, CERN  the rates reach a maximum for 10% CO 2 mixture (see bumps)  maximum and minimum transfer rate gaps over CO 2 concentrations become smaller with increasing pressure  we get smaller rate in the tube with thinner anode wire (blue circles on the plots)  the rates interestingly decrease with increase of CO 2 fraction (first time) !!!

12. Modelling the transfer rates 12/14 MPGD 2013 & 12th RD51 Collaboration Meeting 14– 17 October 2013, CERN  no drops at the highest pressure  there was in Ar – CO 2 mixtures  fit parameters for some mixtures are not physically meaningful  measurements below 0.4 atm may help to get more sensible results for radiative terms  ALICE TPC results does not confirm the decrease with CO 2 concentration  0.5% uncertainty on CO 2 fraction  gain range: 2 10 4 – 5 10 5 Experimental data: C. Garabatos, The ALICE TPC, NIM A 535 (2004) 197–200. Proceedings of the 10 th International Vienna Conference on Instrumentation. Unpublished data for R. Veenhof, Choosing a gas mixture for the Alice TPC, ALICE-INT-2003-29 version 1.0, CERN,2003.

13. Feedback parameters in Ne – CO 2 mixtures 13/14 MPGD 2013 & 12th RD51 Collaboration Meeting 14– 17 October 2013, CERN  both increase of pressure and CO 2 concentration lead decrease on feedback parameters  feedback in Ne – CO 2 mixtures is change broader range compared to Ne – CO 2 mixtures  Ar – CO 2 : 3 – 20 10 -6, Ne – CO 2 : 0.7 – 600 10 -6  calculations on avalanche sizes and photon mean free paths for Ne – CO 2 mixtures are in progress

14. Next 14/14  Hope to publish the results that we have for Ar – CO2 mixtures in very soon  We have last minute experimental gain data for Ne – CO 2 mixtures only 3 days ago; measured in 30% and 50% CO 2 fractions  3.5% CO 2 data will also be ready in a few days (private communication with Tadeusz KOWALSKI, 13th Oct)  very important to fill gaps modelling of the transfer rates  calculations are in progress  measurements in 1% CO 2 would also be very useful !!!  Physical meaning of drops of the transfer rates at high CO 2 fractions has to be worked closely  We will check Magboltz cross sections for Ne by comparing the literature MPGD 2013 & 12th RD51 Collaboration Meeting 14– 17 October 2013, CERN

15. Thanks and ??? MPGD 2013 & 12th RD51 Collaboration Meeting 14– 17 October 2013, CERN